In a typical computed tomography system, an x-ray source, mounted to a rotating gantry, is collimated to form a fan beam with a defined fan beam angle. The fan beam is typically oriented to lie within the "gantry plane` normal to the axis of rotation of the gantry, and is transmitted through an imaged object to an x-ray detector array also oriented within the gantry plane. The axis of rotation of the gantry is also referred to as the z-axis.
The detector array is comprised of a line of detector elements, each of which measures the intensity of transmitted radiation along a ray projected from the x-ray source to the particular detector element. The intensity of the transmitted radiation is dependent on the attenuation of the x-ray beam along that ray by the imaged object.
The x-ray source and detector array may be rotated on the gantry within the gantry plane and around a center of rotation so that the "gantry angle" at which the fan beam axis intersects the imaged object may be changed. At each gantry angle, a projection is acquired comprised of the collected intensity signals from each detector element. The gantry is then rotated to a new angle and the process is repeated to collect projection data along a number of gantry angles to form a 2D tomographic projection set. The acquired tomographic projection sets are typically stored in a numerical form for later computer processing to "reconstruct" a slice image according to reconstruction algorithms known in the art.
As the resolving power of computed tomography methods increases, a growing number of slices are required in the z-dimension. One method of decreasing the scanning time needed to collect such multiple slices of data is to acquire projection data for more than one slice during a given gantry rotation. This may be done by using a two-dimensional detector array extending along the z-axis to obtain projection data on either side of the gantry plane, and by changing the collimation of the x-rays from that of a fan beam to a cone beam having rays diverging from a focal spot not only within the gantry plane but to either side of the gantry plane as well. As with fan beam CT, projection data is acquired at a series of gantry angles about the patient to produce a projection data set. In this case, however, it is a 3D projection data set.
A number of techniques are known in the art for reconstructing images from a 3D projection data set produced by a cone beam. One such technique is described in the paper: "Practical cone-beam algorithm" by L. A. Feldkamp, et al. J. Opt. Soc. Am. A/Vol. 1, No. 6, (June 1984) which is hereby incorporated by reference. As described in co-pending U.S. patent application Ser. No. 093,108, now U.S. Pat. No. 5,400, 377, filed on Jul. 16, 1993 and entitled "Artifact Reduction Method For Tomographic Image Reconstruction Using Cross-Plane Rays", artifacts are produced in the reconstructed images due to the incomplete data. A filtering technique is presented in this co-pending patent application which significantly reduces these artifacts, but it has been discovered that the particular filter proposed therein can be significantly improved to further reduce image artifacts.